Turbine blade repair

ABSTRACT

A method of repairing a turbine blade in which damaged material around a lacing wire hole is removed by machining or chemical etching. New metal is then added by welding such that the lacing wire hole is much smaller or completely eliminated and prior to reforming the lacing wire hole, flats may be formed either by the addition or removal of metal in the region of the lacing wire hole so that a lacing wire hole forming drill or machine bit can enter the turbine blade at a surface substantially normal to its direction of travel, thereby minimizing any tendency of the lacing wire hole forming drill or machine bit to snatch or for the blade to vibrate.

DESCRIPTION OF INVENTION

The present invention relates to a method of repairing turbine bladesand is primarily concerned with repair of rotor blades having aperturestherein through which apertures pass blade interconnecting members suchas lacing wires, cover bands and seals or the like.

For the sake of convenience, all such discontinuities will hereinafterbe referred to as lacing wire holes.

After prolonged use of a turbine, it is not uncommon, particularly withsome designs, for damage to occur around lacing wire holes. In view ofthe inclement environment in which turbines have to operate the metal ofa turbine blade surrounding the lacing wire hole may be adverselyaffected during use. The metal surrounding the hole cane be damaged dueto relative movement between the lacing wire and the blade. Suchrelative movement can cause hardening to the metal of the turbine bladecausing brittleness which can encourage the formation of cracks and, ifno remedial action is taken, any cracks formed may extend further andfurther away from the lacing wire hole into the blade and eventuallyrender the blade in an unsafe condition.

In many cases the condition of the metal surrounding the lacing wirehole may not be immediately apparent. It is envisaged that when aturbine is taken out of commission for a plant maintenance programme,the blades will be inspected and if any blades in the array show anysign of damage it may be that all blades will require attention, albeitthe damage may not be visible.

The damage may occur for a variety of reasons. There is normally aclearance between the lacing wire and the lacing wire hole allowing thelacing wire to float in a dampening mode and the gap that exists cangive rise to the problems mentioned above. Furthermore, if the lacingwire hole is formed with slightly incorrect dimensions this can lead toan increased gap between the lacing wire and the blade, the gap can trapcontaminants and furthermore incorrect fitting between the lacing wireand the blade can lead to movement between the two parts causing ahardening of the metal in that region. The hardening and trapping ofcontaminants leading to the phenomenon known as stress corrosioncracking.

Applicants have been involved in many processes for the repair ofturbine blades, British Patent No. 2,091,139B discloses a method ofrepair of turbine blades in which an outer leading edge part which hasbeen cracked or eroded is removed and replaced by a new piece of hardmaterial. Prior to the application of considerable heat to the bladethat may occur during welding and prior to the application of anyphysical force to the blade, and subsequent heat treatment, any lacingwire hole within the vicinity of the repair such that it may be affectedby the considerable heat or physical force is first filled with weldmaterial. The blade is therefore homogeneous and does not havediscontinuities which, can either during the application of considerableheat during either welding or subsequent heat treatment processes, leadto inbuilt stress in the blade. It has been customary in many repairprocesses carried out by the applicant to first fill the lacing wirehole with weld material and then subsequently reform the lacing wirehole by drilling, for example.

British Patent No. 2,114,921B deals with the same problem and ratherthan filling the lacing wire hole with weld material involves thefilling of a lacing wire hole with a plug which is welded to the blade,the plug may stand proud of the blade or be formed with other formationsto provide locating means for a drilling machine so that the lacing wirehole can be accurately reformed.

A still further process is disclosed in application publication No.2,006,372; the filling of lacing wire holes by driving in a tightfitting plug which can additionally be provided with a heat sink andpossible fluid cooling to minimise adverse affects around the lacingwire hole.

All the above mentioned processes have been concerned with filling thelacing wire hole to remove the discontinuity whilst some other repair isbeing carried out. None however addresses the specific problem of lacingwire holes themselves and possible damage thereto.

It is an object of the present invention to provide a method ofrepairing a turbine blade in the vicinity of a lacing wire hole.

According to the present invention, we provide a method of repairing alacing wire hole in a turbine blade comprising the steps of removingmaterial around the lacing wire hole, replacing the material with newmetal by welding, carrying out a heat treatment process around therepaired area and reforming the lacing wire hole by a machiningoperation.

Preferably more metal is replaced than is removed thereby leaving thelacing wire bole before being reformed considerably smaller than isactually required.

In order to carry out a repair to lacing wire holes, the straightforwardsolution appeared to be merely to enlarge the hole which would removethe damaged metal and replace the lacing wire with one of largerdiameter or by a tube, for example, as is commonly used.

Unfortunately, even though sophisticated machinery had been developed bythe applicant, the re-drilling of lacing wire holes, particularly whenthe blade is in situ on a rotor, is extremely difficult and where it isrequired to removed only a very small amount of material, for example ahardened surface around a lacing wire hole, it has been found to bealmost impossible to accurately re-form the hole to an enlarged size dueto the minimal contact between the machine bit and the blade causing themachine bit to snatch or the blade to vibrate, in all events causingundesired relative movement between the machine bit and the blade.

The above mentioned difficulties have been considerably reduced by theunexpected step of actually replacing the damaged material with morematerial than is required thus considerably reducing the size of thelacing wire hole and ensuring that when re-forming takes place, whetherthe re-formed hole is to be the original size or possibly larger or insome cases smaller, there is more than enough material for engagement bythe machine bit to eliminate or reduce to an acceptable level anytendency to snatch or for the blade to vibrate.

The additional material applied has a secondary advantage in that thediscontinuity left after the welding has taken place and whilst the heattreatment process maybe carried out is very small and, possibly due tothe fact that the heat input to the blade is much smaller than would bethe case for the extensive welding necessary when a new leading edgeouter end portion is welded to the blade, it has been found that in mostcases it is not necessary to completely close the lacing wire hole byweld material, it is however essential to provide sufficient materialfor the machine bit to be able to make proper engagement to reform thehole without any malformation due to snatch or vibration.

The reforming of the lacing wire hole as mentioned above is a fairlydifficult operation and accuracy is of great importance not only thatthe lacing wire hole is correctly positioned but that it is alsoaccurately formed.

One of the main difficulties is "wandering" of the tool reforming thelacing wire hole since the nature of the turbine blade necessitates adrill bit to approach at an oblique angle to the blade. This can causeskidding or other movement of the machine bit since, at least initiallyand at the end of the drilling operation, the force applied to themachine bit is quite uneven.

Preferably, the above mentioned difficulty is overcome by machining aflat on one or both sides of a turbine blade in the region of the lacingwire hole so that a machine bit, for example a drill, approaches thesurface to be drilled to reform the lacing wire hole in a directionnormal to that surface.

Conveniently, the other side of the turbine blade may also be machinedso that on exit of the turbine blade of the machine bit, once again themachine bit is not caused to "wander".

As an alternative to providing a flat on one or both sides of the blade,when a welding operation is applied to the lacing wire hole to replaceany metal removed therefrom, excess metal may be built up on both sidesso that once again the machine bit, for example a drill, can approachand enter the blade to reform the lacing wire hole, the surface of theblade in the region of the lacing wire hole now being normal to theangle of approach of the machine bit.

In a similar fashion the other side of the blade may also be built up sothat during reforming of the lacing wire hole the machine bit is notsubjected to uneven forces.

In order to remove the damaged material, it is envisaged that it will beremoved either by hand tools or machine tools, for example by grinding,and it is also envisaged that the damaged material may be removed by amachining operation.

It will be appreciated however that, as explained above, where it isonly required to remove a small amount of material it may be difficultto accurately remove damaged material by machining due to the machiningbit and the blade. However, since there is to be a subsequent weldingoperation in which the material will be replaced, the accuracy withwhich damaged material is removed, albeit it is of course essential toremove all damaged material, it is not critical.

It is also envisaged that material may be removed by other means, forexample etching with acid particularly where it is necessary to removebraze metal that may be present, where lacing wires or rods or tubeshave been brazed to the blade.

Preferably the method of repair also includes the provision of hardnesstesting after the welding operation such that the heat treatment processcan then be altered so as to properly stress relieve the area around thelacing wire hole without adversely affecting other parts of the blade.

It is also envisaged that the area may be X-rayed to check that alldamaged material has been removed and that no undesired but possiblyhidden cracks or other discontinuities exist.

Preferably, at least when the blade has to be repaired in situ, i.e.whilst still on the rotor and in close proximity to other blades, thelacing wire hole is reformed in accordance with the method and apparatusshown in British Patent Application publication No. 2,240,295.

A method of carrying out the invention will now be described by way ofexample only with reference to the accompanying drawings, wherein:

FIG. 1 illustrates part of a turbine blade having damage around a lacingwire hole,

FIG. 2 illustrates the blade of FIG. 1 with the damaged materialremoved;

FIG. 3 illustrates the blade of FIG. 1 with the enlarged hole partlyfilled with weld metal;

FIG. 4 illustrates part of the blade being subjected to a heat treatmentprocess;

FIG. 5 illustrates the blade with the lacing wire hole re-formed.

FIG. 6 illustrates part of a blade prior to welding of the lacing wirehole;

FIG. 7 illustrates the blade shown in FIG. 6 after welding;

FIG. 8 illustrates a further turbine blade;

FIG. 9 illustrates the blade shown in FIG. 8 after a flat has beenmachined.

Referring first to FIG. 1, a turbine blade is shown at 10 which will beone of a large number of turbine blades in an array and secured to arotor, an adjacent blade is shown at 11 in dotted outline and theblades, or at least groups of blades are often connected by means of alacing wire shown at 12.

The lacing wire passes through a lacing wire hole 13 in blade 10 andwill pass through similar lacing wire holes such as that shown at 14 onthe blade 1 .

The lacing wire 12 may be a wire, a rod or tubular member, its purposebeing to dampen vibration of the blades and/or minimise relativemovement between adjacent blades and in some cases may merely passthrough holes, such as those shown at 13 and 14 in the blades 12 and 11or alternatively nay be secured thereto by brazing, for example.

After some use of the turbine damage in the area of the lacing wireholes 13 and 14 is not uncommon, this is an area of high stress and thelikelihood of damage can be accentuated by inaccurate forming of thelacing wire holes which can lead to gaps between the lacing wire and thesurface of the turbine blade attracting contaminants. Furthermore,slight inaccuracies can also accentuate movement between the lacing wire12 and the blades which can cause local hardening around the surface ofthe blade defining the lacing wire hole and cracks may begin to appear,shown diagrammatically at 15 in FIG. 1.

It may be that whereas cracks may appear around one lacing wire hole butnot around many of the holes in the blades of the array, the fact thatdamage is apparent on a single blade may indicate that it is likely tooccur on other blades in the array even though there is no visible signof such damage. Nonetheless, the hardening of the surface of the bladearound a lacing wire hole, plus the trapping of contaminants in thatarea can lead to premature failure and damage due to stress corrosioncracking.

In order to carry out the repair process, after removal of the lacingwire the lacing wire hole 13 is enlarged to the form shown at 16 in FIG.2, the amount of material being removed depending upon the damage or atleast the extent of the material of the blade that is thought to besuspect.

The material of the blade may be removed by any convenient means and canbe removed by machining or possibly by grinding and with hand tools oreven by an etching process.

In some instances the blade 10 may be removed from the rotor and inother instances in order to carry out a speedy repair, it may remain onthe rotor in which case access to the work site is more difficult.

After the lacing wire hole 13 has been enlarged to the form shown at 16in FIG. 2, material 17 compatible with the blade itself but possiblyincluding other metals alloyed in a manner to provide a durable but longlasting surface is welded to the blade as shown in FIG. 3. It isenvisaged that welding of new metal 17 may take place on either side ofthe blade or possibly both.

Not only is all the metal that has been removed replaced during thewelding process but additional metal 17 is also put in place so that theresulting hole after welding is considerably smaller than the hole 13.

The new metal 17 may be ground or polished so that the concave surface18 and convex surface 19 of the blade 10 are returned to their originalform and then a heat treatment process may be carried out at shown inFIG. 4 in which the area of the blade surrounding the lacing wire holeis heated to a predetermined temperature for predetermined lengths oftime.

FIG. 4 shows a heating mat generally indicated at 20 comprising aplurality of ceramic beads 21 through which heating elements pass and acontrolled current is passed through the heating mat 20 from inlet 22 tooutlet 23.

In order to ensure that the blade is heated to the requiredtemperatures, the temperature of the blade during the heating processmay be carefully monitored by means of thermocouples shown at 24, 25, 26and 27. The thermocouples may be placed at other parts of the blade toensure that those parts of the blade are not adversely affected duringthe welding to provide weld metal 17 or during the heat treatmentprocesses as shown in FIG. 4.

The temperature to which the blade is heated and the length of time forwhich it is heated and held at different temperatures will be dependentupon the extent of heating carried out to replace the weld metal 17 andalso the nature of the blade itself.

It is envisaged that after the welding 17 has taken place that part ofthe blade may be X-rayed and have other tests, e.g. hardness testscarried out thereto to ascertain the most desirable heat treatmentprocess to properly stress relieve the blade.

After the required heat treatment process has been carried out the newlacing wire hole 28 is formed in the blade 10 by a machining operationand the diameter of the new lacing wire hole 28 may be identical to theoriginal hole 13, it may be smaller or larger depending upon the natureof the lacing wire which is intended to be inserted through the hole 28.

Referring now to FIGS. 6 and 7, FIG. 6 shows a turbine blade 30 having alacing wire hole 31. The lacing wire hole 31 is filled with weldmaterial either completely or partially and excess weld material 32 and33 is applied to the blade 30 so that when it is required to reform thelacing wire hole 31 the machine bit, for example a drill, can approach asurface 34 that is substantially normal to the direction of travel ofthe drill during the reforming of the lacing wire hole.

Similarly, the surface 35 from which the drill will exit the turbineblade 30 is also substantially normal ensuring that there is no unevenforces applied to the drill during reforming of the lacing wire hole 31.

An alternative method is shown in FIGS. 8 and 9. A turbine blade 40 inwhich it is desired to have a lacing wire hole 41 is approached by amachine bit, for example a milling cutter 42 in a direction as shown byarrow 43. A flat 44 is milled into the surface of the turbine blade 40thus leaving a substantially normal surface 45 which may be approachedby a lacing wire hole forming tool such as a drill.

If desired, to ensure that the drill connects with the turbine blade 40without any uneven forces being applied thereto, a further flat may beformed in a manner as described above in the region 46.

Whilst the above description relates specifically to a lacing wire holein a turbine blade, it is equally applicable to any other discontinuity,for example apertures provided in the end of the blade which areintended to engage with projections provided on cover bands or otherseals provided at the end of the turbine blade. Such apertures also aresubjected to similar kinds of damage as have been described herein.

The features disclosed in the foregoing description, or the accompanyingdrawings, expressed in their specific forms or in the terms or means forperforming the desired function, or a method or process for attainingthe disclosed result, may, separately or in any combination of suchfeatures, be utilised for realising the invention in diverse formsthereof.

I claim:
 1. A method of repairing a turbine blade, comprising:(a)removing material around a lacing wire hole in the blade; (b) replacingthe removed material with new metal by welding; (c) adding new metal bywelding so as to form an entry surface for a drill or machine bit forreforming the lacing wire hole, and which entry surface is substantiallynormal to a longitudinal axis of the hole; (d) carrying out a heattreatment process on the new metal and a surrounding area of the blade,and (e) reforming the lacing wire hole with said drill or machine bit.2. A method according to claim 1, comprising a further step of adding,by welding, new metal so as to form an exit surface for said drill ormachine bit for reforming the lacing wire hole and which exit surface issubstantially normal to a longitudinal axis of the hole.
 3. A method ofrepairing a turbine blade as claimed in claim 1, further comprisingtesting for hardness the repaired area around the lacing wire hole aftermaterial is added by welding and prior to said heat treatment process,analyzing the information concerning hardness of the repaired areamaterial, and, based on such analysis, determining the heat treatmentprocess conditions necessary to achieve a desired degree of hardness inthe heat treated blade.
 4. A method of repair as claimed in claim 1,further comprising, prior to the reforming of the lacing wire hole,machining a flat on one side of the turbine blade, reforming the lacingwire hole by a drill or machine bit, the drill or machine bit enteringthe turbine blade in the region of said flat.
 5. A method of repair asclaimed in claim 4, further comprising forming two flats on oppositesides of the turbine blade, both in the region of said lacing wire holeor the region where the lacing wire hole is to be reformed, thearrangement being such that the drill or machine bit both enters andleaves the turbine blade while reforming the lacing wire hole through asurface and from a surface substantially normal to a direction of travelof the drill or machine bit.
 6. A method of repairing a turbine blade asclaimed in claim 1, further comprising removing, by a machiningoperation, material surrounding said lacing wire hole which has to beremoved.
 7. A method of repair as claimed in claim 1, further comprisingremoving, by chemical etching, material to be removed around said lacingwire hole.
 8. A method of repairing a turbine blade as claimed in claim1, further comprising X-raying the area of said repair after said repairprocess has been carried out.
 9. A turbine blade repaired in accordancewith the method of claim 1, wherein a longitudinal axis of the reformedlacing wire hole is essentially coextensive with a longitudinalcenterline of the drill or machine bit and substantially free ofwandering from said centerline due to the entry of the drill or machinebit into the replaced weld material in the lacing wire hole at an anglenormal to a horizontal plane extending through the drill or machine bitentry surface.
 10. A turbine blade repaired in accordance with themethod of claim 2, wherein a longitudinal axis of the reformed lacingwire hole is essentially coextensive with a longitudinal centerline ofthe drill or machine bit and substantially free of wandering from saidcenterline due to the entry of the drill or machine bit into thereplaced weld material in the lacing wire hole at an angle normal to ahorizontal plane extending through the drill or machine bit entry andexit surfaces.